1. Field of the Invention
The present invention relates to a motorcycle, and more particularly to a motorcycle having a swing-arm front suspension mechanism.
2. Description of the Relevant Art
There are known motorcycles having a swing-arm front suspension mechanism including upper and lower parallel swing arms supporting a front wheel at their front ends with a front wheel support member such as a knuckle.
For example, U.S. Pat. No. 4,265,329 discloses a frameless motorcycle equipped with a front suspension mechanism comprising upper and lower swing arms.
In the disclosed motorcycle, a joint plate is fixed to the front end of an engine positioned substantially centrally of the motorcycle. The upper swing arm has its base swingably coupled to an upper portion of the joint plate, whereas the lower swing arm has its base swingably coupled to a lower portion of the joint plate. The front ends of the swing arms are pivotally coupled to upper and lower portions, respectively, of a front wheel steering support member on which the axle of the front wheel is rotatably supported and to which a tie rod extending from a steering device is connected. The front suspension mechanism thus constructed also includes an elongate damper having a lower end pivotally coupled to a bracket mounted on an upper portion of an intermediate region of the lower swing arm and an upper end pivotally joined to an upward extension from an upper portion of the joint plate. The motorcycle also has a rear joint plate secured to a rear portion of the engine. A rear wheel is rotatably supported on a rear fork with its front end swingably connected to the rear joint plate.
With the above front swing arm suspension mechanism of the disclosed motorcycle, however, the front joint plate is required and hence the number of necessary components associated with the joint plate is undesirably large. Also, the process of attaching the joint plate to the engine undesirably increases the time required for assembling and servicing the motorcycle. Moreover, the wheel base between the front and rear wheels is increased by the front and rear joint plates through which the swing arms supporting the front wheel and the rear fork supporting the rear wheel are indirectly swingably connected to the engine.
To reduce the wheel base, the joint plates may be dispensed with and the upper and lower swing arms may be pivotally coupled to the engine near its front surface. One such arrangement is employed on a motorcycle disclosed in Japanese Laid-Open Patent Publication No. 53-142739.
The disclosed motorcycle includes a centrally located engine unit comprising an engine and a gear box which are integral with each other. A subframe is disposed on an upper portion of the engine, and a pair of upper and lower front swing arms is coupled to the subframe and the engine. More specifically, the upper swing arm has one end pivotally coupled to the subframe, and the lower swing arm has one end pivotally coupled to the engine unit. The other distal ends of the upper and lower swing arms are connected to upper and lower portions, respectively, of a front wheel support member such as a knuckle by which a front wheel is rotatably supported. The subframe, the engine, the upper and lower swing arms, and the front wheel support member substantially form a parallelogram. A damper has a lower end pivoted to an intermediate portion of one of the upper and lower swing arms and an upper end pivoted to the subframe at a position above the point where the upper swing arm is pivotally coupled to the subframe. The front wheel can be steered by a steering arm coupled to the front wheel support member and connected to a handlebar disposed above the subframe.
This type of motorcycle has a simpler frame and is much lighter than a conventional motorcycle having a head pipe and the motorcycle, described above, which has joint plates.
In the motorcycle of the type described, loads or shocks applied from the front wheel are generally borne by the upper and lower swing arms. With a motorcycle having a low center of gravity, however, it is preferable to have the lower swing arm bear larger loads or shocks than the upper swing arm.
The lower swing arm of the motorcycle disclosed in Japanese Laid-Open Patent Publication No. 53-142739 is pivotally coupled to a bracket projecting from a lower portion of the front area of the engine. For the lower swing arm to be able to bear larger loads or shocks, the bracket must be reinforced for higher rigidity, and hence be of increased dimensions. The bracket of such a structure, however, prevents the engine from being located in a low position and also prevents the motorcycle from having a low center of gravity. The front wheel of the motorcycle is preferably given a positive caster angle in order to permit the motorcycle to run stably in a straight course. In order that the upper and lower swing arms with the positive caster angle on the front wheel extend substantially parallel to each other, the base end of the upper swing arm should be positioned in a central upper area of the motorcycle. Where an L-shaped engine unit having cylinders erected upwardly from a crankcase is mounted on the motorcycle, however, the base end of the upper swing arm must be located in front of the engine since otherwise the base end of the upper swing arm would interefere with the engine cylinders. In order to keep the upper and lower swing arms substantially parallel to each other with the positive caster angle, the base end of the lower swing arm is positioned more forwardly of the engine than the base end of the upper swing arm, with the result that the wheel base becomes excessively long.